Methods to prepare stable electrolytes for all iron redox flow batteries

ABSTRACT

An iron redox flow battery system, comprising a redox electrode, a plating electrolyte tank, a plating electrode, a redox electrolyte tank with additional acid additives that may be introduced into the electrolytes in response to electrolyte pH. The acid additives may act to suppress undesired chemical reactions that create losses within the battery and may be added in response to sensor indications of these reactions.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. 119(e) of U.S.Provisional Patent Application No. 61/778,143, filed Mar. 12, 2013 andentitled METHODS TO PREPARE STABLE ELECTROLYTES FOR ALL IRON REDOX FLOWBATTERIES, the entirety of which is incorporated herein by reference forall purposes.

BACKGROUND AND SUMMARY

The reduction-oxidation (redox) flow battery is an electrochemicalstorage device that stores energy in a chemical form and converts thestored chemical energy to an electrical form via spontaneous reverseredox reactions. The reaction in a flow battery is reversible, soconversely, the dispensed chemical energy can be restored by theapplication of an electrical current inducing the reversed redoxreactions. A single redox flow battery cell generally includes anegative electrode, a membrane barrier, a positive electrode, andelectrolytes containing electro-active materials. Multiple cells may becombined in series or parallel to create a higher voltage or current ina flow battery. Electrolytes are typically stored in external tanks andare pumped through both sides of the battery. When a charge current isapplied, electrolytes lose electron(s) at the positive electrode andgain electron(s) at the negative electrode. The membrane barrierprevents the positive electrolyte and negative electrolyte from mixingwhile allowing ionic conductance. When a discharge current is applied,reverse redox reactions occur on the electrodes. The electricalpotential difference across the battery is maintained by chemical redoxreactions within the electrolytes and can induce a current through aconductor while the reactions are sustained. The amount of energy storedby a redox battery is limited by the amount of electro-active materialavailable in electrolytes for discharge, depending on the total volumeof electrolytes and the solubility of the electro-active materials.

Hybrid flow batteries are distinguished by the deposit of one or more ofthe electro-active materials as a solid layer on an electrode. Hybridbatteries may, for instance, include a chemical that plates as a solidon a substrate throughout the charge reaction and its discharged speciesmay be dissolved by the electrolyte throughout discharge. In hybridbattery systems, the energy stored by the redox battery may be limitedby the amount of metal plated during charge and may accordingly bedetermined by the efficiency of the plating system as well as theavailable volume and surface area to plate.

In a hybrid flow battery system the negative electrode may be referredto as the plating electrode and the positive electrode may be referredto as the redox electrode. The electrolyte within the plating side ofthe battery may be referred to as the plating electrolyte and theelectrolyte on the redox side of the battery may be referred to as theredox electrolyte.

Anode refers to the electrode where electro-active material loseselectrons. During charge, the negative electrode gains electrons and istherefore the cathode of the electrochemical reaction. During discharge,the negative electrode loses electrons and is therefore the anode of thereaction. Therefore, during charge, the plating electrolyte and platingelectrode may be respectively referred to as the catholyte and cathodeof the electrochemical reaction; the redox electrolyte and the redoxelectrode may be respectively referred to as the anolyte and anode ofthe electrochemical reaction. Alternatively, during discharge, theplating electrolyte and plating electrode may be respectively referredto as the anolyte and anode of the electrochemical reaction, the redoxelectrolyte and the redox electrode may be respectively referred to asthe catholyte and cathode of the electrochemical reaction.

One example of a hybrid redox flow battery uses iron as an electrolytefor reactions wherein on the negative electrode Fe²⁺ receives twoelectrons and deposits as iron metal during charge and iron metal losestwo electrons and re-dissolves as Fe²⁺ during discharge. On the positiveelectrode two Fe²⁺ lose two electrons to form two Fe³⁺ during charge andduring discharge two Fe³⁺ gains two electrons to form two Fe²⁺:

Fe²⁺+2e ⁻

Fe⁰ (Negative Electrode)

2Fe²⁺

2Fe³⁺+2e ⁻ (Positive Electrode).

The electrolyte used for this reaction is readily available and can beproduced at low costs (such as FeCl₂). It also has a high reclamationvalue because the same electrolyte can be used for the platingelectrolyte and the redox electrolyte, consequently eliminating thepossibility of cross contamination. Unlike other compounds used inhybrid redox flow batteries, iron does not form dendrites during platingand thus offers stable electrode morphology. Further, iron redox flowbatteries do not require the use of toxic raw materials and operate at arelatively neutral pH unlike similar redox flow battery electrolytes.Accordingly, it is the least environmentally hazardous of all currentadvanced battery systems in production.

However, the above system has disadvantages that limit its practicalityin commercial applications. One of these disadvantages is the lowcycling performance and poor efficiency of these batteries resultingfrom a discrepancy in the pH ranges at which the negative and redoxelectrolytes tend to stabilize. To minimize iron corrosion reactions andto increase iron plating efficiency, a pH between 3 and 4 is desired forthe iron plating reaction. However, a pH less than 1 is desired for theferrous and ferric ion redox reaction to promote redox reaction kineticsand to minimize hydroxide formation.

Concentration gradients across the membrane barrier separating theelectrolytes can cause electrolyte crossover. The Fe³⁺ contaminationfrom the redox side (more acidic) to plating side (less acidic) cancause the formation and precipitation of Fe(OH)₃. This precipitate canfoul the organic functional group of an ion exchange membrane or canclog the small pores of the micro-porous membrane. In either case,membrane ohmic resistance rises over time and battery performancedegrades.

The inventors recognized that the formation of the Fe(OH)₃ precipitatecould be reduced by the addition of chemical chelating agents in theform of organic compounds. These organic compounds could form complexcompounds with Fe³⁺ which has crossed over from redox side to platingside. These complex compounds are soluble in less acidic environment,and thus stabilize the ferric ions. Further, the colors and potentialsof these complex compounds change with solution pH. Therefore, bymonitoring the electrolyte pH via an optical sensor and/orelectrochemical probe, the addition of chemical additives may be meteredso as to achieve and maintain the desired pH in the electrolyte toprevent precipitation and preserve coulombic efficiency.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an example embodiment of the disclosed hybrid flow batterysystem.

FIG. 2 shows a cross section of the disclosed hybrid flow battery systemof FIG. 1.

FIG. 3 is a Pourbaix diagram of Iron ions.

FIG. 4 depicts an example control routine within a hybrid flow batterysystem using the disclosed method.

FIG. 5 illustrates example compounds that formed from Iron and 4different acidic compounds.

FIG. 6 graphically depicts the Fe potential vs. pH of two electrolyteswith different Iron concentrations.

FIG. 7 graphically depicts the Coulombic efficiency to Fe:Acidconcentrations of several acids.

FIG. 8 graphically depicts the Coulombic plating efficiency to pH ratioof 4 different acid additives.

FIG. 9 depicts an example embodiment of an electrolyte probe for thedisclosed system.

FIG. 10 graphically depicts the color of an example electrolyte as afunction of pH.

DETAILED DESCRIPTION

As discussed above, the plating electrolyte used in the all iron redoxflow battery (IFB) may provide a sufficient amount of Fe²⁺ so that,during charge, it can accept two electrons from the negative electrodeto form Fe⁰ and solidify onto a substrate. During discharge, thesolidified Fe⁰ may then lose two electrons, ionizing into Fe²⁺ and bedissolved back into the electrolyte. The equilibrium potential of theabove reaction is −0.44V and thus this reaction provides a negativeterminal for the desired system. On the positive side of the IFB, theelectrolyte may provide Fe²⁺ during charge which loses electron andoxidizes to Fe³⁺. During discharge, Fe³⁺ provided by the electrolytebecomes Fe²⁺ by absorbing an electron provided by the electrode. Theequilibrium potential of this reaction is +0.77V, creating a morepositive terminal for the desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes. Theplating electrode may be coupled to the negative side of a voltagesource so that electrons may be delivered to the electrolyte via theredox electrode. The Fe²⁺ is thus oxidized to Fe³⁺ and may be dissolvedby the electrolyte for subsequent discharge. The electrons provided tothe negative electrode can then reduce the Fe²⁺ provided by theelectrolyte to form Fe⁰ at the plating substrate causing it to plateonto the electrode for discharge.

Discharge can be sustained while Fe⁰ remains in the plating electrolytefor oxidation and the Fe³⁺ remains in the redox electrolyte forreduction. The latter can be maintained by increasing the concentrationor the volume of the electrolyte to the positive side of the battery toprovide Fe³⁺ ions via an external tank of the electrolytic chemical. Thelimiting factor is then more commonly the amount Fe⁰ solidified onto thenegative side of the battery and, consequently, proportional to thesurface area and volume of the substrate that the iron may plate on aswell as the efficiency of plating. Charge is limited by the samemechanism and solidifies as Fe⁰ if ions are available for reduction, itmay similarly be aided by an external tank providing additionalelectrolyte as needed.

In the above reaction the plating electrolyte chemical provides Fe²⁺ andthe redox electrolyte chemical provides Fe³⁺ and Fe²⁺ depending on thecharge state of the system. The use of iron ions in the platingelectrolyte and redox electrolyte provides the ability to use the sameelectrolytic chemical for both sides of the battery, minimizing theelectrolyte cross-contamination that decreases the efficiency of thesystem eventually and leads to an eventual replacement of theelectrolytes. In similar systems, low electrolyte reclamation value canprove an expensive maintenance cost. Further, production of theelectrolyte is cost effective using inexpensive materials such as FeCl₂and FeCl₃.

The electron configuration of iron allows it to solidify into agenerally uniform solid structure on the substrate. In metals commonlyused in similar redox batteries (such as Zinc) the solid structure mayform dendrites during plating. The stable electrode morphology of theIFB increases the efficiency of the battery in comparison to other flowbatteries. Further, no toxic raw materials are used in the battery andit utilizes electrolytes that generally operate at a pH between 1 and 3.Consequently, IFBs are the least environmentally hazardous of advancedbattery systems currently in production.

However, the IFB has several key issues that contribute to performanceand efficiency losses. In particular, battery efficiency losses resultfrom electrolyte crossover through the membrane barrier. Ferric ions inthe redox electrolyte are driven toward the plating electrolyte by theconcentration gradient. Ferric ions that penetrate the membrane barriermay react with the iron metal on the negative side, resulting incoulombic efficiency losses. Ferric ions that penetrate from redox side(more acidic) to plating side (less acidic) can cause the formation andprecipitation of Fe(OH)₃. This precipitation can foul the organicfunctional group of an ion exchange membrane or can clog the small poresof the micro-porous membrane. In either case, membrane ohmic resistancerises over time and battery performance degrades. Additional coulombicefficiency losses can be attributed to 1) the reduction of H⁺ andsubsequent formation of H₂ 2) the H+ ions emitted from the acidicelectrolytes reacting with the plated iron metal to form H₂. The sidereaction can result in hydrogen gassing on the negative side of thebattery during charging.

Fe(OH)₃ precipitate formation resulting from oxidation and ferric ioncrossover can cause barrier fouling. The resulting separator poreblockage may cause high battery ohmic resistance and low cellperformance. Additionally, the redox electrode (Fe²⁺/Fe³⁺ couple) canexperience performance losses over cycles due to a passivating oxidefilm accumulating on the carbon electrode surface.

FIG. 1 shows an example embodiment of an IFB. The plating electrolytemay be stored in plating electrolyte tank 100, the redox electrolyte maybe stored in redox electrolyte tank 101. The plating electrolyte andredox electrolyte may be a suitable salt dissolved in water, such asFeCl₂ or FeCl₃. Both the plating electrolyte and redox electrolyte mayuse the same salt at different molar concentrations, a feature of theIFB not available in batteries with different reactive compounds. Bothtanks may be fluidically coupled to the positive reactor 124 andnegative reactor 122 of the fuel cell. Separating the negative andpositive reactors and their respective electrolytes is barrier 120. Thebarrier may be embodied as a membrane barrier, such as an ion exchangemembrane or a microporous membrane, placed between the redox electrolyteand plating electrolyte to prevent electrolyte cross-over and provideionic conductivity. Sensors 102 and 104 may be used to determine thechemical properties of the electrolyte, including pH and may be embodiedas an optical sensor. Probes 126 and 128 may additionally oralternatively be used to determine the chemical properties (discussedbelow) of the electrolytes. Other embodiments may have a platingelectrolyte probe, plating electrolyte sensor, redox electrolyte probe,redox electrolyte sensor, or some combination thereof. The probe mayalso be placed inside the reacting portion of the IFB in negativereactor 122 and positive reactor 124. The acid additive may be inadditional tank 106 and 108. These may contain different additives andbe controlled by different routines. In other embodiments, the IFB mayalso have either a positive side additive or a negative side additiveand not both. The positive side additive may be accelerated into thepositive reactor 122 by positive additive pump 112; the negativeadditive may be accelerated into the negative reactor 124 by negativeadditive pump 110. Alternately, the electrolyte additives may be pumpedinto tanks 100 and 102. Pumps 122 and 124 may be actuated via a controlsystem communicatively coupled to the pumps. The control system may beresponsive to probe 126, probe 128, sensor 102, sensor 104, or anycombination thereof. The electrolytes may be pumped from the reactor bypumps 130.

FIG. 2 shows a cross section of an example cell of a hybrid all-ironflow battery. The top layer shows the redox plate that may be made ofcarbon or graphite. The redox electrode is adjacent to the redox plateand may be made of graphite. The membrane is immediately adjacent to theredox electrode and the plating electrode and separates electrolytestherein. An example plating electrode may include a substrate structureon which the Fe⁰ may solidify during charging. In IFB's made withmultiple cells, a next redox plate of an adjacent cell may be a backface adjacent to the plating electrode.

Cycling performance losses in the IFB may be attributed to the nature ofthe electrolytes' stability. FIG. 3 shows a Pourbaix diagram that helpsto illustrate the electrolyte stability issue. The vertical axis of FIG.3 represents the voltage potential with respect to the standard hydrogenelectrode, pH is represented on the horizontal axis. During charge, Fe²⁺accepts two electrons to become Fe⁰. However, the reaction competes withthe reduction of H⁺ and subsequent formation of H₂. As a result, theelectrolyte tends to stabilize at a pH range between 3 and 6 on thenegative side of the battery.

During charge, the Fe²⁺ on the positive side of the battery loses oneelectron to form Fe³⁺, an ion with a much lower logarithmic aciddisassociation constant (pKa) than that of Fe²⁺. Therefore, as moreferrous ions are oxidized to ferric ions, the electrolyte tends tostabilize at a pH closer to 1.

Concentration gradients on either side of the barrier during batteryoperation drive an amount of Fe³⁺ over from redox electrolyte to platingelectrolyte. The drastic change in pH from plating electrolyte to redoxelectrolyte (from 1 to 3-6) causes FeOH²⁺ and Fe(OH)₃ species to formand precipitate. These precipitates degrade the membrane by poisoningthe organic functional group of an ion exchange membrane or clogging thesmall pores of the microporous membrane. As the result, the battery'sohmic resistance rises. Precipitate may be removed by washing thebattery with acid, but the constant maintenance limits the batteries usein commercial applications, it also relies upon a regular preparation ofelectrolyte. However, the disclosed method suppresses the abovereactions by adding specific organic acids to the electrolytes inresponse to indications of an electrolyte pH indicative of, andcontributing to, these reactions.

Acidic additive may be added using the example method depicted in FIG.4. The electrolytes may be pumped through their respective electrodeswithin the IFB at 200. At 202 the pH of the battery may be determined inthe electrolyte using a Fe probe to measure the electrolyte potentialvs. a reference electrode, such as Ag/AgCl or H₂ electrode, in theplating electrode. Alternately, the pH may be monitored by measuring thereflective spectra of the electrolyte using an optical sensor via amethod that will be further discussed. Other pH sensing devices nototherwise specified may also be used for the pH determination. Sensorsmonitoring temperature and other operating conditions may also becommunicatively coupled to a control system and used in conjunction withelectrolyte pH within the disclosed method. Note that the informationprovided by these additional sensors may be included in the operationaldefinition of the term “pH” when used as a system control variableherein.

In the disclosed system, the sensors and/or probes may communicate to acontrol system the pH of the electrolyte. If the pH of the platingelectrolyte is found to be above a threshold, such as pH>4, the controlsystem may actuate the release of a preset amount of a prepared acidthat may be added to the plating electrolyte at 204. If the pH of theredox electrolyte is found to be above a threshold, such as pH>1, thecontrol system may actuate the release of a preset amount of a preparedacid to the redox electrolyte. The acid additive added to the negativeand positive sides may be the same or different and may include but arenot limited to hydrochloric acid, boric acid, ascorbic acid, aceticacid, malic acid, lactic acid, citric acid, tartaric acid, isoascorbicacid, malonic acid, glycolic acid, or any combination thereof. Theprocess may return back to 202 to again measure the pH, the process mayrepeat until the pH falls below the threshold. If the pH is below thethreshold the IFB may continue to charge or discharge.

The disclosed embodiment achieves suppression of the aforementionedproblematic reactions by adding specific chemicals (acid additives) tothe electrolytes. The acid additives to the electrolytes may stabilizeFe³⁺ crossover from the redox electrolyte to the plating electrolyte,thus the acid additives used in the embodiment have specific chemicalproperties. Chemical additives that are organic chemicals with shortchains (<6C) and with —OH and/or —COOH groups are sought to stabilizethe ferric/ferrous ions by forming large complexes with these ions. Ashorter carbon chain is sought to minimize the negative effect theseorganic acids may have on overall battery coulombic efficiency becausethese organic acids may have the side reaction of carbon formationduring battery charging. The acids studied for addition and some oftheir properties are listed in table 1 below.

TABLE 1 Organic Acids Tested for Stabilizing IFB Electrolytes Efficiency% Acid Equation pH > 2.5 Carbon g/mol pKa Notes Boric H₃BO₃ 64.0 9.237Reported for H₂ suppression L-Ascorbic C₆H₈O₆ 176.12 4.10 C-A bath, butalso used with Citric acid Glycolic C₂H₄O₃ 93% 0.00% 79.050 3.83 Testingat 10 mA/cm². Grayish dull, rough surface L-lactic C₃H₆O₃ 79% 0.00%90.080 3.86 Testing at 10 mA/cm². Grayish dull, rough surface L-MalicC₄H₆O₅ 90% 0.60% 134.090 3.40 Testing at 10 mA/cm². black and brightsurface. Black oxides precipitated at current densities above 33 mA/cm².Black color was only due to surface film L-Tartaric C₄H₆O₆ >90%  >2.5%150.087 2.95 Testing at 10 mA/cm². burnt. Lots of black precipitatesCitric C₆H₈O₇ 83% 1.00% 192.124 3.09 Testing at 10 mA/cm². black andbright surface. Black oxides precipitated at current densities above 39mA/cm². Black color was only due to surface film Oxalic C₂H₂O₄ 95% 0.13%90.030 1.25 Testing at 10 mA/cm². Grayish dull, rough surface MalonicC₃H₄O₄ 95% 0.13% 104.060 2.83 Testing at 10 mA/cm². Grayish dull, roughsurface Acetic C₂H₄O₂ 95% 0.10% 60.050 4.76 Testing at 10 mA/cm².Grayish dull, rough surface Butonic C₄H₈O₂ 95% .0.15%  88.110 4.82Testing at 10 mA/cm². Grayish dull, rough surface Stinky ErythorbicC₆H₈O₆ 176.18 2.1 In patent as additive

A few examples of the complex structure with ferrous/ferric ions areshown in FIG. 5.

TABLE 2 Organic-Ferrous/Ferric Stability with pH Ascorbic IsoascorbicMalonic pH Acetic Acid Acid Acid acid >2 No No No No PrecipitationPrecipitation Precipitation Precipitation >3 Precipitation No No NoPrecipitation Precipitation Precipitation >4 Precipitation No No NoPrecipitation Precipitation Precipitation

The inventors determined electrolyte stability with these additives andFe plating coulombic efficiencies using an H-Cell setup. Baths wereprepared from reagent-grade chemicals and deionized distilled water thatcontained 0.5 mol/l FeCl₂ and various ratio of one of the organic acids.Bath initial pH ranged from 2 to 3 and they were not adjusted. Agraphite rod was used as the plating electrode and a graphite plate wasused as the redox electrode. Electrodeposition was carried out at aconstant current density of 10 mA/cm². The bath was kept at roomtemperature. The baths were not agitated because bath agitationdecreases current efficiency since the H reduction current attains thediffusion limited current at a more noble potential than Fe depositionand therefore increases with agitation. Current efficiency was evaluatedfrom the weight of the deposits obtained at a given amount of chargeassuming that only Fe was deposited from Fe²⁺; this assumption is validbecause of the relatively low carbon and oxygen content in the deposits.

The equilibrium potentials of an iron surface in IFB electrolytes atvarious solution pH and different Fe-to-organic-acid ratios are shown inFIG. 6. As shown, Fe equilibrium potential decreases slightly between pH1 to pH 4 and then the equilibrium potential rises significantly withpH. The increase is due to thin layers of iron oxide that form on theiron surface at higher pH levels. When running an IFB, if theelectrolyte pH changes from 4 to 5, the battery plating equilibriumpotential could be 50 mV worse, and as the result, the IFB performancecould be 50 mV worse.

The Fe potential as a function of pH is graphically represented in FIG.6. The relationship depicted in FIG. 6 may be used by the control systemto meter pH in the disclosed system. In an embodiment, the controlsystem may measure Fe potential and determine pH using the relationshipdepicted in FIG. 6, or a similar relationship for a corresponding Feconcentration. The pH measurement may be used in the method described inFIG. 4. In other embodiments, the control system may initiate orincrease the addition of an acid additive in order to achieve a desiredFe potential determined by the relationship depicted in FIG. 6.

The coulombic efficiency of Fe plating using these organic acids atvarious ratios is presented in FIG. 7. The relationship presented inFIG. 7 may, in some of the disclosed embodiments, be determinative ofthe chemical composition of the acid additives employed. For example, ifa coulombic plating efficiency above 85% is desired, and malonic acid isused as the acid additive, the control system may maintain a Fe/Acidratio above 20%. The ratio may be maintained via a predetermined maximumamount of acid additive that may be based on the volume and compositionof the electrolyte. As shown in FIG. 7, boric acid, ascorbic acid,L-ascorbic acid, glycolic acid, acetic acid and malonic acid all showhigh Fe plating coulombic efficiencies at high Fe to acid ratios. Asmore acid (lower Fe:acid ratio) were added to the electrolyte, platingcoulombic efficiencies drops. This results from the formation of carbonfrom the organic acids during charging. This chart was used to definethe range of organic acids used in the battery.

Furthermore, the same H-cell tests were performed on several Fe:organicacid ratios to study the crossover ferric ion stability at various pH ofthe plating side as shown in FIG. 8. In some embodiments of thedisclosed system, the results in FIG. 8 were used by the control systemto determine the desired pH of the electrolytic solution to achieve adesired coulombic efficiency. As an example, FIG. 8 graphically depictsthe coulombic efficiency of iron plating for different plating solutionpH levels. As shown in Table 2 and FIG. 8, acetic acid and glycolic acid(not shown) alone cannot stabilize the crossover ferric ion at high pH.However, ascorbic or isoascorbic acid alone is not ideal to be used asthe organic acid because of C formation leading to reduced coulombicefficiency. Carbon formation was detected through electron microscopescanning on iron film plated from a bath with ascorbic acid only.

Therefore, in some embodiments of the disclosed system, combination oforganic acid additives may be utilized to achieve the optimal ironplating bath for performance, efficiency, and stability. In an exampleembodiment, an electrolyte solution of FeCl₂ and NaCl, a first acid(such as boric acid) can be added for H₂ side reaction suppression andhigh coulombic efficiencies. Additionally, a second acid (such asascorbic acid) can be added for ferric ion stability and a third acid(such as glycolic acid) can be added for minimizing carbon formation.

To mitigate electrolyte sensitivity to pH, an example battery may usethe iron potential probe shown in FIG. 9. The probe may be produced witha clean iron wire in conjunction with a reference electrode such as anAg/AgCl wire or a H₂ electrode. The probe can be placed in theelectrolyte tank where Fe potential may be monitored over time. When Fepotential of the electrolyte drifts up, a calculated small amount ofacid can be added to the electrolyte to adjust its pH. By metering theamount of electrolyte additive added in response to the presiding pH,the electrolytes can be more precisely maintained at the ideal pH andcomposition for redox.

The embodiment in FIG. 9 is an Fe potential probe that may be used tomeasure the potential on Fe⁰ and a corresponding pH within the platingelectrode. The potential probe may have inert electrode 206 that may bea wire made of iron or another inert or quasi-inert metal such that theelectrons in the metal will not oxidize or will oxidize at a known rate.Reference electrode 200 may be a wire containing silver (Ag) and an Agsalt such as AgCl or a H₂ reference electrode. For example, in anembodiment, the Fe probe may be placed in the plating electrolyte tomeasure solution potential on Fe. Further, Fe may represent the solutionpotential and pH for the plating side of the battery. The electrodes maybe electrically isolated from each other by insulator 202 that may bemade of any material with low or no conductivity. Heat shrink 200 actsto keep the Fe and the reference electrode at a set distance.

In other embodiments of the disclosed system, the pH of the electrolytesmay also be monitored by a sensor that may be used independently, or incombination with, the probe. In an embodiment, the optical sensor maymeasure the absorption spectrum of ambient light through the liquid todetermine the corresponding pH. The optical sensor can also be used tomonitor battery state of charge if chelating organic acid is added tothe electrolyte to increase iron ion stabilities. This is becausechelated iron complex shows different color at different pHs. Forexample, if ascorbic acid is used as the chelating agent, the ironsolution color goes from green to violet and then black from pH of 2 topH of 6.

The control system communicatively coupled to the sensor may determinethe pH using the pH to color relationship depicted in FIG. 10. Here thepH-color relationship depicted graphically, in which the vertical axisrepresents the average number of H+ bound per carbon atom and thehorizontal axis is a logarithmic representation of h. As shown, at lowpH (higher number of H+ bound per C) the solution is green or pale, asthe number of free H+ increases (increasing pH) the solution becomesviolet and eventually black when the average number of free H+ ishighest. By measuring the wavelength of ambient light or a light from aknown source through and/or reflected by the electrolyte, the pH of theelectrolyte may be determined.

In an example embodiment, white light may be incident on the surface ofthe electrolyte. A spectroscope may be utilized within the sensor todetermine the wavelength of light reflected by the electrolyte. If areflected and/or transmitted wavelength is found to be, for instance,less than 450 nm (corresponding to a violet hue) acid additive may beadded to the solution to lower the electrolyte pH. Further, thespectroscope may continue to monitor the absorption spectra of theelectrolyte and if the reflected and/or transmitted wavelength is foundto be above a threshold, such as 510 nm (corresponding to a green hue),the addition of acid additive may be terminated.

On the negative side of the IFB, during charge Fe²⁺ accepts twoelectrons and forms Fe⁰. The competing reaction on the negative side ofthe battery (H⁺ accepts one electron and forms H₂) results in thetendency of the electrolyte on the negative side of the IFB to rise overcycles from pH of 2 to pH of 6 thus embodiments of the disclosed systemmay use the probe and sensors above to monitor pH change.

As shown previously in FIG. 6, the pH change may result in up to 100 mV‘apparent’ performance loss of the battery due to Fe equilibriumpotential drifting up with higher pH level. To mitigate performanceloss, an embodiment of the disclosed Fe Potential Probe or opticalsensor, such as those described above, may be used to monitor batterystate of charge as well as the electrolytes' pH level.

The operation window for the plating electrolyte of the battery isbetween pH of 3 and 4. Therefore, in an embodiment, when either a FePotential Probe or an optical sensor shows a pH level above 4, a small,pre-calculated amount of acid may be added to the plating electrolytesolution to return the plating electrode to an optimal pH range. As aresult, the battery performance may be stabilized.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology may be applied to otherflow battery types. The subject matter of the present disclosureincludes all novel and nonobvious combinations and subcombinations ofthe various systems and configurations, and other features, functions,and/or properties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application.

Such claims, whether broader, narrower, equal, or different in scope tothe original claims, also are regarded as included within the subjectmatter of the present disclosure.

1. An iron redox flow battery system, comprising: a redox electrodefluidically coupled to a redox electrolyte tank; a plating electrodefluidically coupled to a plating electrolyte tank, wherein a platingelectrolyte is fluidically coupled to a plating electrolyte acidadditive or a redox electrolyte is communicatively coupled to a redoxelectrolyte acid additive; a pH monitoring device monitoring a pH of theplating electrolyte or redox electrolyte; and a control system withinstructions to pump an amount of plating electrolyte additive or redoxelectrolyte additive in response to the pH monitoring device.
 2. Thesystem of claim 1, wherein the plating electrolyte or redox electrolyteinclude FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, or any combination thereof. 3.The system of claim 2, wherein the plating electrolyte or redoxelectrolyte include NaCl, NH₄Cl, LiCl, Na₂SO₄, (NH₄)₂SO₄, Li₂SO₄, or anycombination thereof.
 4. The system of claim 1, wherein the pH monitoringdevice is a Fe potential probe comprising a clean iron wire inconjunction with a reference electrode such as Ag/AgCl electrode or anH₂ electrode.
 5. The system of claim 1, wherein the plating electrolyteadditive, redox electrolyte additive, or both include boric acid,ascorbic acid, acetic acid, malic acid, lactic acid, citric acid,tartaric acid, isoascorbic acid, malonic acid, glycolic acid, or anycombination thereof.
 6. The system of claim 1, wherein platingelectrolyte, redox electrolyte or both include chelating organic acid.7. An iron redox flow battery operating method, comprising measuring thepH of an plating electrolyte, redox electrolyte, or both and adding acorresponding amount of plating electrolyte additive to the platingelectrolyte and a corresponding amount of or redox electrolyte additiveto the redox electrolyte.
 8. The method of claim 7, comprising addingthe plating electrolyte additive, redox electrolyte additive, or both inresponse to a pH measurement above
 4. 9. The method of claim 7,comprising determining the pH with a Fe potential probe that includes areference electrode such as Ag/AgCl or an H₂ electrode.
 10. The methodof claim 7, comprising determining the pH with an optical probe.
 11. Themethod of claim 11, comprising measuring an absorption spectra of theplating electrolyte, redox electrolyte, or both.
 12. An iron redox flowbattery system, comprising: a redox electrode fluidically coupled to aredox electrolyte tank; a plating electrode fluidically coupled to aplating electrolyte tank, where the plating electrolyte and redoxelectrolyte are coupled to a plating electrolyte additive and redoxelectrolyte additive respectively; a pH monitoring device monitoring apH of the plating electrolyte or redox electrolyte; and a control systemwith instructions to pump an amount of plating electrolyte additive orredox electrolyte additive in response to the pH monitoring device